Published German Patent Application No. 10 2008 030 473 A1 discloses a method for determining contact points of a decoupling clutch in the drivetrain of a hybrid vehicle. Published German Patent Application No. 10 2008 027 071 A1 discloses a method for determining coefficients of friction of a decoupling clutch in the drivetrain of a hybrid vehicle.
To adapt the clutch parameters, such as coefficient of friction and contact point position, a known model-based approach like that depicted in FIG. 1 is used. With the aid of model 100, an estimated engine speed and an estimated wheel rotation speed are determined from engine torque 110, which is assumed to be known. In control engineering, this approach is referred to as an observer if errors 140, 150, which are obtained from the comparison of the estimated speeds with measured speeds 120, 130, react on drivetrain model 100 in such a way that errors 140, 150 decay from their initial value exponentially toward zero. The decay performance of the error can be represented by a time constant, and corresponds in the broadest sense to a filtering of the coefficient of friction determined by the observer. Thus, the latter cannot follow the real coefficient of friction of the clutch at an arbitrary speed. On the contrary, the attempt is made to obtain a relatively stable coefficient of friction through compensatory measures. FIG. 1 depicts the powertrain model of the observer with the reduction of the calculated wheel/engine speed error.
With a rapid change in the real coefficient of friction, which cannot be greatly attenuated through a compensatory measure, the coefficient of friction is determined falsely for a certain time interval. If the coefficient of friction in the software is clearly too high in comparison with the real coefficient of friction of the clutch, then the torque delivered by the engine will lead to a rapidly increasing engine speed, which in return results in a great increase in the friction. In the opposite case, the clutch will catch immediately, rendering vibration damping due to slippage at the clutch impossible.
In the case of high friction values—for example greater than 25 kW—it has been found that the coefficient of friction rises slightly at first. This increase was taken into account by friction compensation on the coefficient, but afterward the real coefficient of friction drops off sharply. Since the instant at which the drop occurs varies greatly, this effect cannot be softened by means of a compensatory measure. At the same time, the greatly increased engine speed results in another great energy input, so that the coefficient of friction of the clutch is also unable to increase. This effect can be detected in varyingly severe manifestations in all dual clutch systems.